Electronic digital signal can be converted to optical digital signal for use in an optical communication network.
FIG. 1. shows an apparatus 100 for converting an electronic digital signal to an optical digital signal. The apparatus 100 uses a direct current laser 106 and an electro-absorption modulator 102 to transmit optical signals in an optical communication system 110. A steady laser beam is directed by the laser 106 onto the electro-absorption modulator 102. The electro-absorption modulator 102 modulates or permits less or more of the laser light to pass therethrough. The electro-absorption modulator 102 is controlled by a control signal, such as a digital electrical signal. When a high voltage difference is applied across the electro-absorption modulator 102, more light is absorbed by the modulator and less laser light is permitted to pass therethrough into the optical communication network 110. Conversely, when a low voltage difference is applied across the electro-absorption modulator 102, less light is absorbed by the modulator and more laser light is permitted to pass therethrough. By modulating the amount of laser light according to the control signal, the electro-absorption modulator 102 converts the electrical signal into an inverted optical signal.
In some implementations, it is important to know the optical power of the transmitting laser when transmitting an optical signal in an optical communication system 110. This is because the optical power measurement is used in a feedback loop to help keep the optical power output of the laser constant. Laser optical power output is liable to change due to voltage variations used to power the laser (bias), the laser's temperature, and the laser's hours of operation (age).
The optical power of the laser 106 is measured by either tapping and measuring a portion of laser energy from the back of the laser 106 using a back-facet monitor 150, tapping and measuring a portion of the laser energy from the front of the laser using a front-facet monitor 152, or by splitting a portion of the modulated laser light away from a transmit path into a sense detector using a power tap 154. Power tapping is performed by splitting a small part of the optical signal away from the main optical path and diverting into a photodetector 156. The photodetector 156 outputs an analog signal which is digitized by an analog-to-digital converter 158. For ease of reference, all three of these methods are shown together in the apparatus 100 shown in FIG. 1. For clarity, any one of these methods can be used alone or in conjunction to measure the optical power of the laser 106.
All three of the methods for measuring optical power involve taking a portion of the energy or light from the laser 106 by splitting the laser beam and using a photodetector 156 and an analog-to-digital converter 158 to measure the optical power of the laser beam. Since the laser beam is split, all three methods also result in a loss in the transmit optical power of the laser. To compensate for the loss of optical power, electrical power to the laser is increased. Greater electrical power, however, creates more heat which can further degrade the optical power of the laser. If significant heat is generated by the laser, then cooling may also be required.
Improvements in measuring the optical power of a laser during modulation are desirable. Namely, the improvements would reduce the amount of laser energy used to measure the optical power of the laser.